DEVELOPMENT. 



The mesoderm, although at first a solid mass of cells, 

 is an invaginated fold in which a cavity soon appears, 

 having an inner layer of cells that affiliates closely 



Fig. 8. Sagittal section through an egg of triton (after the end of 

 gastrulation) : ak, outer germ-layer; ik, inner germ-layer; dz, yolk- 

 cells; dl and vl, dorsal and ventral lips of the ccelenteron; ud, coelen- 

 teron; d, vitelline plug; mk, middle germ-layer (Hertwig). 



with the hypoderm cells, and an outer layer that ap- 

 plies itself to the ectoderm. The hypoderm with 

 its mesoderm layer is known as the splanchnopleure, 

 while the ectoderm and its mesoderm is the so- 



Neural groove. 



Entoderm, Notocord. 

 Fig. 9. Transverse section of chick embryo 22 hours old. 



matopleure. The new cavity thus produced is the 



